PTFE molded articles are excellent in heat resistance, resistance to chemicals and low frictional properties and have been widely used as chemical plant parts, domestic goods with a non-tacky coat, medical parts, etc.
In recent years, PTFE molded articles have been attracting attention as a sliding part integrated into a hydraulic device or a compressor, such as a bush, a piston ring, a seal ring, etc.
Conventional processes for producing a PTFE molded article include compression molding comprising charging PTFE powder in a cylindrical mold and pressing the powder from one direction at room temperature to bond the powder to each other (see PLASTIC ZAIRYO KOZA (6) FUSSO JUSHI (Fluorine Resin), pp. 58-69, Nikkan Kogyo Shinbunsha (1981)) and isostatic molding comprising charging PTFE powder between a mold and an elastic mold and pressing the powder by fluid pressure from every direction via the elastic mold to bond the particles to each other. The isostatic molding is also disclosed in Japanese Patent Publication No. 43-21984. The accepted molding pressure to be used in compression molding and isostatic molding usually ranges from about 100 to 1,000 kg/cm.sup.2 and from about 50 to 1,000 kg/cm.sup.2, respectively. Either of these techniques involves a sintering step in which the molded article is heated to a temperature above the melting point of PTFE. The resulting sintered molded article may be cut to shape and size by means of a lathe, etc.
It is preferable for some of the above-described uses that a PTFE molded article should have the same or approximately the same coefficient of thermal expansion in every direction. If the coefficient of thermal expansion of a molded article greatly varies depending on the direction, the molded article expands to a specific direction with a rise in temperature when used as an integral part of a hydraulic device or a compressor, causing abnormal friction or reduction in sealing performance. Further, a PTFE molded article is demanded to have frictional resistance for some uses.
However, a molded article obtained by compression molding is ought to have a largely varying coefficient of linear expansion depending on the direction (i.e., anisotropy), which is attributed to the unidirectional pressing. The frictional resistance of the molded article is not so high as expected.
On the other hand, a molded article obtained by isostatic molding is preferred as having the same or approximately the same coefficient of linear expansion for every direction (i.e., isotropy) but still has room for improvement in frictional resistance.